Okay, so hopefully this should make it clear.Let's define X as...

Okay, so hopefully this should make it clear.

Let's define X as the set of values which define when you were born (without the first two digits as your specified).

So X is: X = {00, 01, 02, 03, .... 98, 99} (all the way up to 1999, let's leave it there).

Now let's define Y as your age:

So Y is: Y = {00, 01, 02, 03, ... 99} (all the way up to 99 years old).

Now there has to be a RELATIONSHIP between X and Y. It's currently 2011, so the year you were born in has to be: 2011 - Y. But remember that we're truncating the year to the last two digits so it's actually:

X = 2011 - Y - 1900 (we're subtracting 1900 because when we've truncated the year 19** to being simply ** we're effectively subtracting 1900).

evaluating for X: X = 111 - Y

This can be rearranged to X + Y = 111

So no matter what you do, it always adds up to 111. Cheap party trick, nothing more =D